This invention is related to a device according to the precharacterising part of enclosed claim 1. In its preferred embodiment, the device forms a robot, the second element of which is adapted to carry, directly or indirectly, a working member. This working member may serve for arbitrary movement, processing, working etc. of objects.
For moving and rotating objects without changing the inclination of the objects robots of the so called SCARA type are primarily used today. These robots are manufactured for the. four degrees of freedom x, y, z and xcfx86z (rotation of the object about an axis parallel to the z-axis). For manipulating the object in the xy-plane, two arms coupled in series are used, said arms working in the xy-plane, the axes thus being perpendicular to the xy-plane. In order to achieve movement in z-direction a linear movement device is used. This device may be present either after the arms coupled in series or before the arms coupled in series. In the first case the arms coupled in series must move the drive assembly for the z-movement and in the latter case the drive assembly for the z-movement must move the arms coupled in series. The drive assembly for the xcfx86z-movement will always be located at the extreme end of the kinematic chain of the robot.
In applications where lighter objects are to be moved with requirement of very short movement times, SCARA robots, as well as all other robots having kinematic links coupled in series, present the disadvantage that performance is restricted by the moving mass of the robot itself. This makes SCARA robots to obtain a lower limit as to the cycle time for e.g. an application where objects are to be continuously picked and placed. This cycle time as short as possible is determined primarily by the thermal load of the motors. In order to reduce the cycle time, a larger motor torque and, accordingly, larger motors are required which results in a larger moving mass of the robot. This means in its turn requirements for more rigid arm systems, which resuits in an even larger moving mass etc. A forced motor cooling may of course be used, transmissions be introduced to displace the motors more closely to the base of the robot etc. but this simultaneously makes the robot more expensive and less robust.
A possibility to drastically reduce the moving mass of a robot is to use a parallel robot structure. A study of which arm structures are possible for parallel robots is made in the article xe2x80x9cStructural kinematics of in-parallel-actuated robot-arms) of Prof. K. H. Hunt (Journal of mechanisms, transmissions and automation in design, December 1983). Of particular interest is an arm structure consisting of three parallel link arrangements, where each link has five joints, four joints of which form two joint pairs of cardan type. This link type is described in detail in the article xe2x80x9cConstantvelocity shaft couplings: a general theoryxe2x80x9d of Prof. K. H. Hunt (Journal of engineering for industry, May 1973). An implementation of a robot using the kinematics structure described by Prof. Hunt is shown in U.S. Pat. No. 4,976,582. For movement of objects in x-, y- and z-direction, three force applying arrangements are used in the form of drive means, which are located in a triangularly shaped pattern on a first element in the form of a stationary platform and which are connected to a movable element in the form of a further platform, on which the working member, e.g. a gripping means, is mounted via a link arrangement of their own having five joints. One of these joints is realised by rotatability. of an output axle from the drive means whereas the four other joints are implemented as either two cardan joints, which in an embodiment with two mutually hingedly connected arm components of the link arrangement connect one of these arm components with the remaining arm component and with the second element respectively, or as four cardan couplings or four spherical joints in case one of the arm components in the link arrangement comprises two links arranged in parallel. To rotate the object, a transmission between a drive means on the stationary first element and a rotation device on the movable second element is used. This transmission must provide five degrees of freedom, which more specifically are realised in the form of a translation coupling and two cardan joints.
Since all drive means are mounted on the stationary first element, the parallel robot structure will have a considerably smaller movable mass than a SCARA robot. This makes it possible to achieve much shorter cycle times at the same available drive means torque. However, the parallel arm robots suggested by Prof. Hunt and described in the U.S. Pat. No. 4,976,582 have several disadvantages relative to SCARA robots:
1. The working range is small relative to the volume required by the arm structure of the robot.
2. The robot cannot turn around to serve several working surfaces in the surroundings of the robot.
3. To work above a working surface, the stationary first element of the robot must be suspended in a frame. This causes several problems:
3.1 Additional costs for the frame
3.2 The frame makes it more difficult to reach the working area of the robot.
3.3 The frame has a mechanical natural frequency which is excited by the robot and which causes lack of accuracy when positioning the objects moved by the robot.
3.4 Oil from bearings and gear boxes and dirt collected on the first stationary element of the robot will fall straight down into the working area of the robot.
3.5 The robot is more difficult to reach for repair and service.
3.6 A longer cable assembly is required, both for robot connection and gripping means connection and possibly for forced motor cooling.
Thus, a parallel robot structure, which does not have all the above disadvantages relative to a SCARA robot, would be desirable. This means that one would require a parallel arm robot with SCARA kinematics. Such a robot kinematics is described in the European patent EP 0 668 130 B1. The movable element is there formed as a platform and moved by means of two arms having five joints and one arm with three joints. The arms with five joints work in the xy-plane and use an arrangement with parallel lines to obtain a constant inclination of the movable platform. The arm with joints works in the xz-plane and contributes to locking of all degrees of freedom of the movable platform. However, this SCARA-similar robot structure has the following disadvantages compared to the parallel robot structure in the U.S. Pat. No. 4,976,582:
1. The arms most closely to the movable platform must transfer torque from the movable platform to the stationary platform. This means that bearings and arms must be dimensioned accordingly, which results in a larger moving mass than in the case where three pairs of parallel links, which only have to absorb normal forces, are used.
2. The motor manipulating the arm with three joints must be turned around and this results in an extra moving mass of the robot.
3. The arrangement with parallel lines to obtain a constant inclination of the moving platform has a shorter life and less accuracy than parallel links. Besides the lines and their attachments will gather dirt and make it impossible to use the robot in applications having requirements with regard to hygiene.
A parallel robot with a SCARA-similar kinematics but not presenting the disadvantages mentioned above is described in the Swedish patent 9700090-5. Three drive means are there mounted with coinciding rotation centra and each drive means is connected to the movable element (platform) via arms having five degrees of freedom. The arms most closely to the movable platform consist of three, two and one link respectively and these links only have to transfer normal tensions between the movable and stationary elements (platforms). This makes the arm system very light and at the same time very rigid. Besides, the joints are only subjected to a normal force and the bearings may therefore be made small with a resulting small moving mass of the robot. All drive means are mounted on the stationary element (platform) (FIG. 10) and since they have a common axis centre, the whole robot may be turned around by synchronous control of the drive means.
A disadvantage with the robot structure according to the Swedish patent 9700090-5 is that one of the arms has three links between the movable element (platform) and an arm part mounted on an output axle of the drive means and that another arm only hat one link. This results in the links for these arms not being possible to be mounted in pairs, which otherwise is an advantage, both from the point of view of force transfer and with regard to bearing design. One could, of course, introduce redundant links so that the arm with three links obtains two link pairs. and the arm with one link obtains one link pair, but this results in a larger moving mass and increases the costs for the robot.
The present invention has for its object to reduce the above discussed problems of the parallel robot structure in the Swedish patent 9700090-5.
The object stated is according to the invention achieved by providing the device with the features appearing from the enclosed claims, and then primarily claim 1. A robot is thereby obtained with link arrangements operating in parallel, said robot being well suited for floor mounting, and presenting, in addition, SCARA-kinematics and having a minimum of movable mass. This robot provides great possibilities for picking and placing objects to a low cost and with very short movement times. Examples of applications are sorting and packaging of cakes and pralines, handling of small packages, cans and bottles and packages of tablets and tablet charts within the pharmaceutical industry.
Advantageous developments of the basic concept of the invention are defined in the dependent claims 2-19.
Further advantages and features of the invention appear from the enclosed description.